1. Field of the Invention
The present invention relates generally to acoustic imaging systems.
2. Related Art
Various imaging techniques for imaging inside objects are known. For example, x-ray and magnetic resonance imaging has found use as a medical diagnosis tool. It is also known that acoustic waves can be propagated through objects. When an acoustic wave propagates through a non-homogenous object, it is affected by spatial variations in the stiffness, density and absorption of the object. Non-uniformities can result in attenuation, varying propagation speed, reflection, refraction, and scattering of the acoustic waves passing through the object. By transmitting an acoustic wave into the object, and measuring the resulting affect on the acoustic wave by the object, it is possible to construct an image of the internal structures of the object. This image construction process is sometimes called “inverse scattering.”
While many advances in image imaging have been provided over the years, solutions to specific problems have achieved varying success. For example, the present inventors have been developing the technique of ultrasound computed tomography for several years. Ultrasound computed tomography may enable imaging structures within the human breast to allow diagnosis of diseases, such as breast cancer.
Other than skin cancer, breast cancer is the most common cancer among women, and is the second leading cause of cancer death in women, after lung cancer. Approximately 44.5 million women in the United States are screened for breast cancer each year with 10% or 4.5 million referred for a second diagnostic test. The American Cancer Society Breast Cancer Statistics report indicates that 1 in 7 women will get breast cancer during her lifetime. The current standard of care has significant problems, generating unacceptably high rates of false positive tests—between 8% and 10%—and upwards of 15% false negative tests. The result is that many women suffer from unnecessary and invasive biopsies. In addition, each year the U.S. healthcare system spends an estimated $2.1 billion on biopsies, which yield negative results more than 75% of the time.
Breast cancer is a malignant tumor that has developed from cells of the breast. A malignant tumor is a group of cancer cells that may invade surrounding tissues or spread (metastasize) to distant areas of the body. The female breast is made up mainly of lobules (milk-producing glands), ducts (milk passages that connect the lobules to the nipple), and stroma (fatty tissue and connective tissue surrounding the ducts and lobules, blood vessels, and lymphatic vessels). Lymphatic vessels are like veins, except that they carry lymph instead of blood. Lymph is a clear fluid that contains tissue waste products and immune system cells (cells that are important in fighting infections). Lymph nodes are small bean-shaped collections of immune system cells that are found along lymphatic vessels. Cancer cells can enter lymphatic vessels and spread to lymph nodes. Most lymphatic vessels in the breast connect to lymph nodes under the arm (axillary lymph nodes). Some lymphatic vessels connect to lymph nodes inside the chest (internal mammary nodes) and either above or below the collarbone (supra- or infra-clavicular nodes). When breast cancer cells reach the axillary (underarm) lymph nodes, they may continue to grow, often causing the lymph nodes in that area to swell. If breast cancer cells have spread to the underarm lymph nodes, they are more likely to have spread to other organs of the body as well. Thus, it is important to find out if breast cancer has spread to the axillary lymph nodes when choosing a treatment.
Most breast lumps are not cancerous, that is, they are benign. Most lumps turn out to be fibrocystic changes. The term “fibrocystic” refers to fibrosis and cysts. Fibrosis is the formation of fibrous (or scar-like) connective tissue, and cysts are fluid-filled sacs. Fibrocystic changes can cause breast swelling and pain. This often happens just before a period is about to begin. The breast may feel nodular, or lumpy, and, sometimes, a clear or slightly cloudy nipple discharge is noticed. Benign breast tumors such as fibroadenomas or papillomas are abnormal growths, but they are not cancer and cannot spread outside of the breast to other organs.
Although widespread use of screening mammography has increased the number of breast cancers found before they cause any symptoms, some breast cancers are not found by mammography, either because the test was not done or because even under ideal conditions mammography cannot find every breast cancer. The most common sign of breast cancer is a new lump or mass. A painless, hard mass that has irregular edges is more likely to be cancerous, but some rare cancers are tender, soft, and rounded. An elliptical mass with its major axis perpendicular to skin line or to natural internal tissue planes (or penetrating such planes), is a danger signal that requires further investigation. For this reason, it is important that a health care professional who is experienced in diagnosing breast diseases check any new breast mass or lump.
Other signs of breast cancer include a generalized swelling of part of a breast (even if no distinct lump is felt), skin irritation or dimpling, nipple pain or retraction (turning inward), redness or scaliness of the nipple or breast skin, or a discharge other than breast milk. Sometimes a breast cancer can spread to underarm lymph nodes even before the original tumor in the breast tissue is large enough to be felt.
If there is any reason to suspect breast cancer, other tests should be performed. After a complete physical exam (including a clinical breast exam), doctors often recommend a diagnostic mammogram or a breast ultrasound. A clinical breast examination (CBE) is an exam of the breasts by a health professional, such as a doctor, nurse practitioner, nurse, or physician assistant. The examiner first looks at the breasts for changes in size or shape. Then, using the pads of the finger tips, the breasts are felt for lumps.
Although mammograms are mostly used for screening, they can also be used to examine a breast of a woman who has a breast problem. This can be a breast mass, nipple discharge, or an abnormality that was found on a screening mammogram. In some cases, special images known as cone views (or spot views) with magnification are used to increase contrast and thus make a small area of altered breast tissue easier to evaluate. A diagnostic mammogram may show that a lesion (area of abnormal tissue) has a high likelihood of being benign (not cancer). In these cases, it is common to ask the woman to come back sooner than usual for a recheck, usually in 4 to 6 months. On the other hand, a diagnostic mammogram may show that the abnormality is not worrisome at all, and the woman can then return to having routine yearly mammograms. Finally, the diagnostic work-up may suggest that a biopsy is needed to tell if the lesion is cancer.
Ultrasound, also known as sonography, uses high-frequency sound waves to outline a part of the body and to visualize internal organs and tissue. High-frequency sound waves are transmitted into the area of the body being studied and echoed back. A computer or dedicated electronic circuitry picks up the sound wave echoes and changes them into an image that is displayed on a computer screen. Breast ultrasound is sometimes used to evaluate breast abnormalities that are found during mammography or a physical exam. One of the most common abnormalities that women have is fibrocystic disease. Ultrasound is useful for detecting fibrocystic disease. It is the easiest way to tell if a cyst is present without placing a needle into it to draw out fluid. It can also find some breast masses. Conventional medical ultrasound uses a single ultrasound array to both transmit and receive echoes and thereby measure the ultrasound reflectivity and distance of various objects under the skin surface. It assumes that the speed of sound is constant through the tissue being imaged. It has difficulty imaging objects with low reflectivity or high absorption of sound. It produces images which are typically two-dimensional, distorted, grainy, and contain speckle. Foreground objects tend to mask deeper structures (e.g. cast shadows on).
A biopsy is done when mammograms, ultrasound, or the physical examination finds a tumor. A biopsy is the only way to tell if cancer is really present. All biopsy procedures remove a tissue sample for examination under a microscope. There are several types of biopsies, such as fine needle aspiration biopsy, core (large needle) biopsy, and surgical biopsy. Biopsies are often done under ultrasound or MRI guidance. Each type of biopsy has distinct advantages and disadvantages. The choice of which to use will depend on the specific situation. Some of the factors the doctor will consider include how suspicious the lesion appears, how large it is, where in the breast it is located, how many lesions are present, other medical problems the patient may have, and the patient's personal preferences. Statistically, three of four biopsies are benign.